Appliance running cost calculator

Every appliance carries a wattage somewhere — moulded into the plug, printed on a plate at the back, buried in the manual — and that number on its own tells you almost nothing, because what costs money is watts multiplied by time. A 2000 W kettle used for four minutes a day is cheap. A 40 W router left on for all of them is not, and it is the second one people are usually surprised by. This page takes the wattage, the hours you actually use it and the price on your own bill, and turns them into a figure per day, per month and per year. It ships no tariff of its own: electricity prices differ by country, by supplier and often by the hour, so the only rate worth using is the one you type in.

How it is calculated

kWh = W × h ÷ 1000 · cost = kWh × price per kWh

A watt is a joule per second and an hour is exactly 3600 seconds, so a kilowatt held for an hour is 3.6 megajoules — one kilowatt hour, the unit your meter counts. Divide watts by a thousand to get kilowatts, multiply by the hours the appliance runs, and multiply the result by your price per unit. The yearly figure scales the daily one by the days you say it runs; the monthly figure is a twelfth of the year, not thirty days of it.

Source: NIST SP 811 (2008), Table 2, SI coherent derived units — watt (W), the SI unit of power: 1 W = 1 J/s

Questions people ask

Where do I find the wattage?

On the rating plate, usually at the back or underneath, next to the voltage. If only amps are given, multiply amps by the mains voltage where you live to get watts. Watch for a plate that quotes the peak draw rather than the typical one — a fridge is the classic case, since its compressor cycles on and off and the plate describes the compressor running rather than the day as a whole. Where the energy label prints a yearly kWh figure, that already accounts for the cycling, so trust it over the wattage.

Why does it not know my electricity price?

Because there is no such thing as the electricity price. It differs by country, by supplier, by tariff and increasingly by the hour of the day, and a number written into this page would be wrong for most people the day it was written and wrong for everyone a year later. Your bill has the figure, usually per kWh or per unit. Type that and the answer is about you rather than about an average.

Does this include the standing charge?

No, and it should not. A standing charge is a daily fee for being connected at all; you pay it whether this appliance exists or not, so adding it here would make every appliance look more expensive than it is. What this page gives you is the marginal cost — what changes on the bill if you use the thing or do not.

What does "days a year" do?

It separates things that run all year from things that do not. A fridge is 365; a patio heater might be 30 and an air conditioner 90. The daily figures are for a day the appliance actually runs, and the yearly figure multiplies them by the days you set. The monthly figure is then a twelfth of that year, so for a seasonal appliance it is an average across the whole year rather than a real month of use.

Does standby power matter?

It adds up more than people expect, because it runs for the other twenty-odd hours. A device drawing 2 W on standby uses about 17.5 kWh over a year, which is small but not nothing, and a house with twenty of them is running a permanent 40 W load. If you want that figure, enter the standby wattage with 24 hours a day rather than trying to fold it into the active figure.

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